Project Summary/Abstract The physiological function of the aryl hydrocarbon receptor (AHR) is an area of keen interest, especially in terms of its role in gut homeostasis, microbiome composition, and immune regulation. In addition, absence of AHR expression in the gastrointestinal tract leads to increased susceptibility to an adverse challenge, such as citrobacter rodentium exposure. Perhaps this is due in part to the central role that the AHR plays in Il-22 expression within the gut. We have published a number of articles identifying endogenous or microbiome generated AHR ligands, many of which have been discovered to be dramatically more potent activators of the human AHR compared to mouse AHR. These studies have lead to the central hypothesis that the human AHR plays an increased role in gut homeostasis compared to the mouse AHR. In order to test this hypothesis, we need to develop a suitable humanized AHR mouse. Thus, the specific aim is to generate a knock-in mouse that expresses the human Ah receptor. The actual hypothesis would be addressed in a subsequent grant application. The mouse line will be generated by Cyagen; a company with extensive experience with generating knock-in mice. The key aspect of our approach is the use of a codon optimized and secondary RNA structure disrupted synthetic human AHR cDNA, which exhibits a 4-fold higher level of expression than wild-type human AHR cDNA. The knock-in human AHR mouse line will be characterized to establish that it exhibits a similar level and pattern of AHR expression relative to a C57BL/6J mouse. In addition, the level of induction of AHR target genes is appropriate for the ligand specificity of the human AHR previously established. This humanized AHR mouse will be useful for a wide range of studies of the role of the AHR in disease processes, as well as assessing the human AHR as a therapeutic target.